1. Field of the Invention
The present invention relates to a protection cover and, more particularly, to a protection cover used in varistors.
2. Description of Related Art
Common metal oxide varistors (MOV) can be classified into three categories: SiC, Si Zener diode and ZnO. Ceramic bipolar varistors formed by sintering ZnO powder at high temperatures have a nonlinear resistance-versus-voltage relation. They have a very high resistance that can block the passage of current at low voltages, but their resistance drops drastically to allow the passage of large currents at high voltages. This characteristic has been widely applied in the voltage regulation and the suppression of transient surge of electronic circuits and power systems such as communication equipments, consumer electronic products, industrial machines and power distribution and transmission devices.
When the MOV continually receives a high voltage or the received voltage exceeds the bearable rated value, a large amount of heat will be generated or even fire is caught. In an electronic circuit, temperature has very high influence to the properties of electronic components. High temperature may cause abnormal operations of the circuit or even cause the circuit not to work, not mentioning the influence of fire to the electronic components.
The influence to the circuit of high temperature caused by the MOV that is damaged by high voltages can usually be reduced through the protection of a temperature breaker. The temperature breaker is adhered onto the surface of the MOV to detect the temperature of the MOV. If the MOV is damaged to cause a rise in temperature or to further catch fire, the temperature breaker will cut off the power of the circuit to avoid hazards once detecting the high temperature.
The adhesion between a common MOV and the temperature breaker, however, is not good enough. There exist gaps between them. The temperature breaker cannot accurately detect the temperature because of the existence of gaps. The situation that the temperature breaker does not work when the temperature is high enough to influence the circuit may arise. Besides, when the temperature breaker is adhered onto the surface of the MOV, if the MOV is burned out instantaneously under a high voltage, the temperature breaker may also be burned out instantaneously and therefore cannot cut off the working power, hence being not able to exactly protect the circuit.